1. Technical Field
The present invention relates generally to electronic devices, and in particular to enclosures for electronic devices. The invention relates particularly to enclosures that consume minimal space and provide structural stability to internal electronic devices. More particularly, the invention relates to enclosures which prevent the leakage of electromagnetic radiation. Still more particularly, the present invention relates to electronic circuit card enclosures and tailstock bracket support structures that promote electromagnetic interference shielding while providing rigid support to electronic circuit cards surrounded by such enclosures.
2. Description of the Related Art
In the electronics and computer industry there exists a continuing need to provide sufficient shielding against electromagnetic radiation leakage. The challenge to designers of electronic and computer devices has been to design enclosures that provide structural integrity to the internal device, such as logic cards, while maintaining the components of the internal device in as compact an arrangement as possible. Logic cards, also referred to as circuit cards, can be described as information-carrying media, common to most computers, for the introduction of data and instructions into the computers either directly or indirectly.
It is critical today to shield electronic equipment such as computer and computer hardware, against electromagnetic interference (EMI) emissions. In the past, computer products contained slower processors. Consequently, the problem of escaping emissions of these types through openings in computer enclosures and logic card enclosures was not significant. But the advent of faster processors and switching circuits has altered this situation. While increased processing speeds have made remarkable improvements in performance and proficiency, EMI containment problems have been exacerbated.
Today, it is not uncommon for computers to employ clock speeds in the range of 50 to 200 MHz or higher. Furthermore, digital circuits operating at these high frequencies may have pulse rise times of a nanosecond or less. These high speeds can cause EMI to be radiated at harmonic frequencies in excess of one GHz. The Federal Communications Commission ("FCC") has established standards that limit the amount of allowable EMI emissions from electronic devices. To meet FCC emissions standards it is necessary to seal around expansion slots holding electronic modules such as logic cards and to seal around slots covered by blanks when these slots are not being used. Such blanks are often referred to in industry as "fillers." In addition, commercial pressure exists to compact the spacing between expansion slots or openings.
To attenuate EMI, electronic modules need to be more properly grounded about their perimeter. Electronic modules are typically packaged next to one another with grounding springs required to be placed between the modules being required. Installing springs on the electronic modules requires special module handling as the delicate springs are easily damaged. A typical electronic module utilized in industry is a logic book, which essentially includes a logic card enclosed within the logic book. Logic books typically plug into a mechanical structure known as a "cage," which maintains one or more logic books in place. Such circuit books generally are utilized in association with computers or computer workstations. Internal components of such computers or computer workstations are susceptible to electromagnetic radiation from logic cards through gaps in the logic book.
Present designs for circuit or logic cards include internal tailstock brackets contacted by "book" or card structure covers, well known in the art of logic card design. The term "tailstock" is traditionally utilized in the art of mechanical engineering to describe a part of a lathe that holds the end of the work not being shaped, allowing it to rotate freely. As applied to the art of logic cards and logic books, the term "tailstock" typically refers to a rear bracket that provides input and output exit ports and EMI shielding to the logic book. Because of the inherent lack of stiffness of existing logic card cover designs, including those which utilize tailstock-type brackets, manufacturing tolerances (i.e., flatness, hole spacing, etc.) and the limitation of fasteners located on the sides of such cards, results in the formation of long gaps between the tailstock bracket and the book covers. Equivalent attenuation of electromagnetic radiation requires shorter slots or gaps with newer cards because such newer cards utilize higher frequency and faster clock speeds. Previous designs also have been designed and constructed with less conductive material which reduces conductivity across joints to a point which, while acceptable to these older designs in association with other contact devices, have an impaired electromagnetic radiation performance when utilized in association with higher frequency cards.
To prevent problems associated with typical logic cards, such as electromagnetic radiation, some sort of card enclosure must be provided. Such an enclosure should be conductive in order to create a Faraday cage, and should not have large openings or slots which may provide entry or exit points for electromagnetic radiation. A Faraday cage, also known as a Faraday shield, is an electrostatic shield, shaped like an enclosed metal box. The Faraday cage or shield can be composed of wire mesh. Typically, a Faraday cage is an enclosed metal box. The higher the speed or frequency of particular logic cards, the smaller any openings, gaps, or slots must be within the logic book in order to provide the same amount of radiation attenuation. Present logic books are designed as Faraday cages and are adequate for the relatively slower clock speeds and frequencies associated with cards presently utilized in industry, but are not adequate for the newer faster frequencies and clock speeds inherent with more advanced logic cards.
From the foregoing it can be seen that a need exists for a logic book enclosure that prevents electromagnetic radiation leakage, while providing rigid structural support to logic cards enclosed within the logic book. From the foregoing it can also be seen that a need exists for logic book enclosures that provide improved grounding. This can be achieved by providing a logic book configuration that maintains intimate contact between the logic card and all grounding and shielding hardware, while also providing shorter gap lengths and positive ground contacts. Present devices do not achieve these goals. A need thus exists for a device which would achieve these goals.